1. Field of the Invention
The present invention relates to a protection circuit, and more particularly to a protection circuit which protects a load circuit operating with power supplied from an AC adapter and which has a soft start function.
2. Description of the Related Art
A consumer product operating with a DC power source often receives DC power supply from a commercial power source through an AC adapter. When an AC adapter other than a designated AC adapter is connected for DC power supply, predetermined DC power is not supplied to cause overvoltage, overcurrent, or low voltage, which disables a normal operation of the consumer product. In some cases, the consumer product might be damaged.
For example, JP-A-11-18280 describes an overvoltage detection circuit in an electronic device to which a power source is supplied with an AC adapter, the overvoltage detection circuit using a zener diode, a resistor, and a transistor to prevent a supply of overvoltage due to a difference in specification of AC adapters.
Some consumer products have a protection circuit having a soft start (or soft switch) function, as well as an overvoltage protection circuit. According to the soft start function, a consumer product can receive a power supply after a predetermined delay time from a point at which an AC adapter is inserted, whereby a sharp voltage change upon the insertion of the AC adapter can be eased.
For example, JP-A-2010-220277 describes, as an abnormal voltage protection circuit, a configuration including a power supply detection circuit having a zener diode that is turned on when power with a supply voltage of 15 V or higher is supplied from an input terminal to which an AC adapter is connected; an overvoltage detection circuit having a zener diode that is turned on when power with a supply voltage of 18 V or higher is supplied; and a microcomputer controlling a power supply to an electronic circuit connected to an output terminal based on the detection results of these two detection circuits.
In this configuration, the microcomputer is operated with an internal power source such as a button battery. When the power supply detection circuit detects a voltage of less than 15 V, the microcomputer controls not to supply power to the output terminal, and when the power supply detection circuit detects a voltage of 15 V or higher, the microcomputer controls to start an action of a counter for a soft start. After the counter indicates a lapse of a predetermined time, the microcomputer controls to start a power supply to the output terminal, but when the overvoltage detection circuit detects a voltage of 18 V or higher during the power supply, the microcomputer resets the counter. Specifically, this publication describes that, when power with a supply voltage within a range of 15 V to 18 V is supplied from the input terminal, power is supplied to the output terminal after a lapse of a predetermined time from the power supply detection.
Some consumer products have incorporated therein a DC/DC converter for adjusting a voltage value of DC power supplied from an AC adapter to an optimum voltage value. A DC/DC converter temporarily receives power supplied form an AC converter, adjusts a voltage value of the power to an optimum value, and then, supplies the resultant power to a load circuit. Thus, a supply of overvoltage or the like can be prevented.
JP-A-09-284994 points out that, when a DC-DC converter, which is provided between an AC adapter and a load circuit and has a function as an overvoltage protection circuit to the load circuit, is broken down, the DC-DC converter loses the function as the overvoltage protection circuit, and therefore, overvoltage is likely to be supplied to the load circuit. In view of this, a fuse is provided after the AC adapter, and an FET is connected in series between the fuse and the DC-DC converter to apply a voltage at a load-side terminal of the DC-DC converter to a gate of the FET. When the voltage at the load-side terminal becomes an overvoltage due to breakdown of the DC-DC converter, the FET is shut off to prevent a supply of overvoltage to the load.
In a consumer product having a soft start function, an internal circuit is not operated until an AC adapter is connected. Therefore, one of the problems is at which timing the soft start function and an overvoltage detection function and the like are performed. JP-A-2010-220277 aims to solve this problem by the microprocessor having a button battery. However, this technique cannot be applied to a consumer product not having a button battery incorporated therein.